Use of liquid-crystal display devices is expanding year by year as energy-saving and space-saving image display devices. Heretofore, one serious defect of liquid-crystal display devices is that the display image viewing angle dependence of the devices is large. Recently, however, wide viewing angle liquid-crystal display modes such as VA-mode and the like have become put into practical use, and accordingly, even in the market of televisions and others that require high-quality images, the demand for liquid-crystal display devices is rapidly expanding now.
The basic constitution of the liquid-crystal display device comprises a liquid-crystal cell with a polarizer arranged on both sides of the cell. The polarizer plays a role of transmitting a light polarized in a predetermined direction alone, and the performance of a liquid-crystal display device greatly depends on the performance of the polarizer therein. The polarizer generally comprises a polarizing element with a transparent protective film stuck to both sides thereof, in which the polarizing element is formed of a polyvinyl alcohol film or the like having adsorbed iodine or dye through alignment thereon. A cellulose acylate film of typically cellulose acetate has high transparency and can readily secure strong adhesiveness to polyvinyl alcohol used as the polarizing element, and is widely used as a polarizer protective film.
It is known that disposing a retardation film between the polarizer and the liquid-crystal cell in a liquid-crystal display device realizes wider viewing angles, or that is, improves display characteristics. As the retardation film, a cellulose acylate film is specifically noted that can express excellent optical properties, concretely excellent in-plane retardation Re (nm) and thickness-direction retardation Rth (nm) of the retardation film; and such a cellulose acylate film is used in a liquid-crystal display device as the retardation film therein.
Some additives are known, which are added to cellulose acylate film to improve the performance of the film. As one known compound usable as an additive to cellulose acylate film, there may be mentioned a retardation enhancer. Patent Reference 1 discloses, as a retardation enhancer, a compound capable of forming a molecular complex that contains a keto-enol tautomerizable compound as the constituent element thereof, concretely disclosing, as one example thereof, a compound that has a 1,3,5-triazine ring-containing structure such as a guanamine skeleton or the like. Similarly, Patent Reference 2 discloses, as a compound capable of increasing Rth of cellulose acetate film when added thereto, a retardation enhancer comprising a discotic compound. The retardation enhancer disclosed in this patent reference is a discotic compound, and, as one example thereof, disclosed is a structure containing a 1,3,5-triazine ring or a porphyrin skeleton.
With the recent tendency toward expanding use of liquid-crystal display devices, use of those devices for large-size and high-definition televisions and others has become expanded, and the requirements for the quality of polarizer, retardation film and polarizer protective film are much increasing. In particular, large-size and high-quality liquid-crystal display devices are desired to be used in various severer environments than before. From such viewpoints, the cellulose acylate film for use in liquid-crystal display devices is earnestly desired to satisfy both the requirement for resistance to optical performance change depending on the environmental humidity and the requirement for protection of polarizer in use in high-temperature and high humidity environments.
However, already-existing cellulose acylate films, including those containing, as added thereto, a retardation enhance having a specific structure as in the above-mentioned Patent Reference 1 or 2, could not sufficiently satisfy both the requirement for resistance to optical performance change depending on the environmental humidity and the requirement for protection of polarizer in use in high-temperature and high humidity environments.
Given the requirements as above, for example, Patent Reference 3 discloses a method of using a cellulose acylate film having a specific degree of acyl substitution to thereby enhance the resistance to optical performance change depending on the environmental humidity and enhance the resistance to performance change in use in high-temperature and high humidity environments.
Regarding the method of enhancing the resistance to performance change depending on the environmental humidity and enhancing the resistance to performance change in use in high-temperature and high humidity environments, by adding additive to cellulose acylate film, Patent Reference 4 discloses a method of using a cellulose acylate film that contains a polyester, and a polyalcohol ester or an aromatic-terminated ester.